Stir together matzo meal and 1/2 teaspoon salt in a bowl. Make a well in center and add eggs and yolk, then lightly beat eggs with a fork. Add water and oil and stir until dough just comes together. Knead dough 5 or 6 times. Form into a 5-inch square and put on a work surface dusted with potato starch, then dust dough with potato starch. Let stand 10 minutes.
Meanwhile, dust a large baking pan with some potato starch. Quarter dough. Roll out each piece on a surface dusted with potato starch until as thin as possible (1/16 inch thick) and about 10 by 8 inches, dusting rolling pin and surface of dough as needed. (If dough tears, patch it.) Using a pizza wheel or sharp knife, cut into long 1/2-inch-wide strips and transfer to pan in 1 layer along with any broken pieces of dough.
Cook in a pasta pot of boiling salted water (2 tablespoons salt for 6 quarts water) until just tender, 1 to 2 minutes. Reserve 1 cup cooking water, then drain pasta in a colander.
Heat oil in a 12-inch heavy skillet over medium-high heat until it shimmers, then sautée asparagus, zucchini, peas (if using), white and pale green parts of scallions, and 1/2 teaspoon salt, stirring occasionally, until vegetables begin to soften, about 5 minutes.
Add 1/2 cup reserved cooking water to vegetables and cook, shaking skillet occasionally, until vegetables are just tender, about 2 minutes. Stir in pasta and scallion greens until just coated with sauce. Remove from heat and stir in cheese and zest. Stir in more reserved water if desired. Serve with additional parmesan.
